WASHINGTON -- Before the opening statements got under way in the impeachment trial of President Clinton yesterday, Chief Justice William H. Rehnquist delivered an important message to the Senate chamber. It had nothing to do with the Constitution, or the impeachment of Andrew Johnson or the future of the U.S. presidency. No, it was about something else:

His bad back.

"I would like to inform members of the Senate and the parties in this case of my need to stand on occasion to stretch my back," he said solemnly. "I have no intention that the proceedings should be any way interrupted when I do so."

The chief justice has been troubled by back problems for more than two decades, and he tries not to sit for long periods. Yesterday, he got up and stretched repeatedly.

In the Supreme Court, Rehnquist is well known for leaving the bench about once an hour and disappearing behind a curtain to pace and take a break from his black high-backed chair.

The 74-year-old justice suffered a lower back injury while playing table tennis in 1977 and since then has been hospitalized at least three times for problems stemming from back pain.

Rehnquist underwent a relatively common surgery in 1995 to remove a herniated disc and relieve compression of his spinal cord. His back and leg pain have improved since then, a Supreme Court spokesman said, but he still stretches and walks regularly.

Orthopedic surgeons say a Senate trial is not an ideal setting for back sufferers.

"For degenerative disc disease, sitting is harder on the spine than standing," said David Johnson, an orthopedic surgeon at Washington Hospital Center. "To get up and walk around a bit gives the disc a little bit of a vacation.

"It would be even better if the justice were lying down. Or the best would be if they could move the proceedings to the Senate pool."

The private pool in the Senate gym will not be offered anytime soon, so Rehnquist will have to cope. The Senate took an extended break about once every two hours yesterday.

The chief justice, who walks stooped over with his left shoulder markedly higher than his right, suffered acute pain in 1977. He was hospitalized for a week and put in traction after playing table tennis and going for a one-mile run. He missed court for three weeks.

Several years later, Rehnquist ran into trouble with a medication that helps pain sufferers sleep -- the tranquilizer Placidyl, usually recommended at bedtime for two-week periods to combat insomnia.

In 1981, veteran Supreme Court reporters noticed Rehnquist slurring his speech, hesitating and mispronouncing words. Ultimately, the drug was blamed for the speech problem, which observers said did not affect the justice's keen intellect.

Later, the Washington Post quoted a medical report showing that between 1977 and 1981, the justice frequently went through three-month doses of Placidyl in only a month. Court observers said Rehnquist's mental powers remained as ever in this period.

The justice went off the Placidyl in late December 1981 and was hospitalized in 1982 after reportedly suffering serious withdrawal reactions. At the time, George Washington University Hospital officials told reporters that Rehnquist had suffered "some distorted perceptions" and "some awarenesses that were not real" after going off the drug.

The medical treatments for public figures often are scrutinized, and Rehnquist is no exception. Yesterday, pain management experts questioned why he was ever placed on Placidyl.

"That's a step up from phenobarbital -- we're talking Stone Age medicine," said Dr. Nelson Hendler, an assistant professor of neurosurgery at the Johns Hopkins School of Medicine. "There were plenty of other drugs other than Placidyl to take in the 1970s."

As Rehnquist handles double duty in the Supreme Court and in the impeachment trial, veteran court watchers say he shows no signs of strain. He reportedly continues his daily walks; in the past he has played tennis and gone swimming to help his back.

As he presides over a chamber meant to represent the will of the people, Rehnquist himself seems to speak for America. After all, 60 percent to 80 percent of the population suffers serious lower back pain later in life, according to Georgetown University Medical Center statistics.

"It's the human condition," said Johnson. "The chief justice is no different from most of us."